Field of the Invention
This invention relates to a rolling stand for calibrating or reducing rolling mill with multiple rolls for tubes made of steel or other metal.
State of the Art
Calibrations made with known calibrating or reducing rolling mills for steel tubes or rounds have the feature of having an ovalization of the outer surface intended as ratio between the space left free for the body being processed in the zone of the gap between the adjacent rolls, since that zone is usually also called gap zone, generally indicated with H2, and the space left free for the body being processed at the groove bottom zone of the roll, generally indicated with H1. This happens at each roll, irrespective of how many rolls the stand is currently made of, for example 2, 3, or 4 rolls.
According to the prior art, the angular sector of the roll comprised between the groove bottom zone and the gap zone has a distance H(α) increasing as a function of α, α being the angle with the central vertex on the rolling axis Y and having line B as a side passing by the bottom zone of the roll. FIG. 1 shows an example of four-roll calibrating rolling stand of the prior art.
The rolling mills of this type are normally of the multi-stand type, wherein the stands are in a succession along the rolling axis Y, with decreasing calibration section making sure that the groove bottom zones of the stands in odd positions match the gap zones of the stands in even positions and the groove bottom zones of the stands in even positions match the gap zones of the stands in odd positions, irrespective of the number of rolls making up each stand.
In the general case, the working sector of each roll is equal in degrees to αroll=360°/NR where NR indicates the number of rolls per stand.
Therefore, for stands with 2 rolls, the working sector has an angular width αroll=360°/2=180°,
for 3 roll stands αroll=360°/3=120°,
for 4 roll stands αroll=360°/4=90°, and so on as NR increases.
Accordingly, the offset angle between odd and even stands becomes β=αroll/2, i.e.
for 2 roll stands β=180°/2=90°,
for 3 roll stands β=120°/2=60°,
for 4 roll stands β=90°/2=45°.
FIG. 2 shows the case of two consecutive stands of the prior art projected on the same section plane, with NR=3, offset by angle β=60°.
FIG. 3 shows a quadrant of the cross section of a rolling roll with a stretch S of the roll surface in a polar reference system and FIG. 4 shows the pattern of the same surface S of the roll in a projection in a Cartesian axis reference system. Therefore, the function representing the calibration profile Rpass=H(α) is generally an even function with a relative minimum for α=0° and a maximum value in the gap zone.
The last stand of the rolling mill usually has a perfectly round section to eliminate any shape defects in the tube or round section that may be found after the passage of the tube or round in the previous stands.
Rolling practice and theoretical simulations confirm that the material squeezed radially towards the center by the groove bottom zones of the rolls of each stand tends to overfill in the gap zones. This trend is more marked as the number of rolls per rolling stand decreases and the ratio between nominal diameter and thickness of the tube wall increases. In particular, it has been seen that with the recent introduction of four roll stands in the rolling mills, the material of the tube pushed towards the center Y along four directions angularly offset at 90° from each other tends, on the other hand, to shrink also in the gap zones. This phenomenon is easily understood since the angular sector comprised between one push point and the next one in the circumference direction is reduced and therefore, the material of the tube or round is more guided during the deformation thereof.
The prior art rolling mills generally provide for a more oval-like calibration set, i.e. with larger ratios H2/H1 for thin tubes and smaller H2/H1 for large tubes, which forces to have a large number of calibration roll sets available, increasing the cost of a rolling mill.
Document U.S. Pat. No. 3,842,635 discloses a rolling stand with three rolls for the cold rolling of tubes by means of a mandrelmandrel. Each roll of the stand has two relative minimums of the roll surface radius at an angle Φ measured by the line passing by the groove bottom zone of the roll and by the rolling axis. Such groove profile is recommended for reducing rolls that must be in any case followed by finishing rolls that completely transform the section of the outer surface of the tubes which takes on a complex, non-circular shape, for example triangular or hexagonal. This document does not address the problem of achieving a perfectly circular final section tube shape.
An attempt of making the final profile of a rolled tube more circular at the end of a sequence of thickness reductions preventing the forming of a polygonal inner section of the tube and the elimination of overfilling in the gap zones has been made in patent EP1707281 discloses a solution with a succession of rolling stands with rolls having the groove profile with a variable radius which increases starting from a minimum radius at the line passing by the groove bottom zone by the rolling axis. The radius increases gradually or in portions up to reaching the maximum at the gap. In practice, the theoretical contact between the roll bottom and the outside of the roll is arranged at the groove bottom. In this solution there is only one relative minimum of the radius of the roll groove surface. This profile has a bending always directed towards the same side along the whole groove profile. This solution seems more suitable when the tubes have a thicker wall while it is not optimal for rolling tubes with a thinner wall.
While these solutions offer final tube sections that achieve high quality, they do not always meet the market requirements that requires top quality rolled material, such as tubes and rounds, with as small number of reduction and calibration stands as possible.